Crater Shape Research Articles

Rapid fabrication of a microdevice with concave microwells and its application in embryoid body formation

Here, we report a novel method for the fabrication of polydimethylsiloxane microdevices with complicated 3-D structures, such as concave and crater shapes, using an easily machined polymethyl methacrylate mold combined with a one-step molding process. The procedure presented here enables rapid preparation of complex 3-D microstructures varying in shape and dimensions. To regulate embryoid body (EB) formation, we fabricated a microfluidic device with an array of concave microwells and found that EBs growing in microwells maintained their shape, viability, and a high degree of homogeneity. We believe that this novel method provides an alternative for rapid prototyping, especially in fabricating devices with curved 3-D microstructures.

The origin of crystalline residues in Stardust Al foils: Surviving cometary dust or crystallized impact melts?

Abstract– Samples returned by the Stardust mission from comet 81P/Wild 2 provide an unequaled opportunity to investigate cometary formation and evolution. Crystalline silicates have been identified in impact craters in Stardust Al foil, yet their origin is ambiguous. They may be original cometary components, or they may have grown from melt generated by impact. We have now studied experimental impacts of the calcium silicate mineral wollastonite, using scanning and transmission electron microscopy to document the relationship between impact feature shape and crystal lattice orientation in impact residue. Wollastonite can have a characteristic acicular habit, forming crater shapes that indicate crystal orientation upon impact. From extracted impact residue, we determined the lattice orientation of crystalline material for comparison with the whole particle orientation. We assume that crystallization from melt, without surviving seed nuclei, should result in randomly oriented crystallite growth, with no preferred direction for individual crystals. However, we find that the majority of crystalline material in the residue retains b‐axis orientation parallel to the long axis of the crater form. This, together with impact parameter calculations and lack of Al incorporation by the residue (suggesting melting did not occur), indicates that these crystals and, by analogy, the majority of Al‐free crystalline silicates in Stardust foil, are surviving remnants of the impactor. Furthermore, amorphous wollastonite residue probably did not form via melting and subsequent quenching, but instead by high‐pressure amorphization or degradation of unquenchable phases. Finally, one crystal studied appears to be a new high‐pressure/temperature polymorph of CaSiO3, indicating that such polymorphs may be observed in Stardust residues in craters.

Wire electrical discharge machining applied to high-speed rotating workpieces

In order to evaluate the influence of high-speed rotating workpieces on wire electrical discharge machining (WEDM), single discharge experiments were carried out inside a grinding machine, in a self-designed wire electrical discharge dressing device (WEDD-device). The shape and size of eroded craters, measured on the workpiece/anode, were found to be highly influenced by the applied relative speed. Based on the crater's shape, its radial expansion speed can be calculated and the slip of the plasma arc column can be measured. Additionally, it was found that the volume of eroded craters increases as relative speed is increased, indicating that higher melting efficiencies are achieved for higher relative speeds. Finally, an electro-thermal model is described and simulation results are discussed, which help to better understand the influence of relative speed on erosion.

Effect of impact angles on ejecta and crater shape of aluminum alloy 6061-T6 targets in hypervelocity impacts

The effect of the impact angle of projectiles on the crater shape and ejecta in thick aluminum alloy targets was investigated in hypervelocity impacts. When polycarbonate projectiles and aluminum alloy 6061-T6 target were used, the impact angle of the projectiles clearly affected the crater shape, as expected. The impact angle also affected the ejecta mass, ejecta size and scatter angle. However, the effect at 15 ∘ and 22.5 ∘ was not great. When the impact angles were 30 ∘ and 45 ∘ , the effect was clearly confirmed. The impact angle clearly affected the axial ratio of ejecta fragments, c / a .

ZnO Etch-Feature Control via Concentration and Temperature of Various Acids

Separating the dependence of ZnO:Al thin-film texturization from deposition conditions is an important aspect for optimization of ZnO as light scattering, transparent contact in solar cells. The crater density and shape for a given polycrystalline ZnO:Al thin-film are shown to be controllable by altering the temperature and concentration of various acids. The crater densities generally increase with increasing temperature, decreasing concentration, or by etching in a weak acid with small molecular size. The etching trends observed for polycrystalline ZnO:Al films are confirmed on ZnO single crystals. We discuss in detail the etch process in terms of etch rates and crater formation statistics. Results are explained in accordance with a recently proposed ZnO etch model, and possible physical explanations are given.

Surface modification of monocrystalline zinc oxide induced by high-density electronic excitation

Strong modifications of semiconductors can be provoked by high-density electronic excitation. We report on surface structuring of monocrystalline wurtzite O-face (0001) ZnO excited by UV femtosecond laser pulses (248 nm) below the ablation threshold. At fluences above 11 mJ/cm2, nanoholes of D=10 nm diameter appear quasi-periodically separated by a distance ∼30 nm (=3 D). Dual-pulse (pump-pump) experiments permit estimation of the electronic excitation lifetime responsible for this nanostructuring, which is in agreement with the electron-hole plasma lifetime 220 ps. The nanostructuring results in a smaller monocrystalline domain of ∼0.1 μm size and increases the crystalline interplane c-distance by 0.11%. The excitonic luminescence of the irradiated sample is found to increase by about 10 times. The nanostructuring remains stable in a limited range of laser fluences: above 40 mJ/cm2 the surface melts, which accelerates the photoinduced bonds breaking leading to surface erosion. We tentatively ascribe the related mechanism to the nucleation-growth of cluster vacancies at crystal dislocations accelerated by the non-thermal (electronic) melting of the surface layer. At fluences lower than 11 mJ/cm2, larger volcano-like features of 60-nm diameter were observed. The characteristic crater shape and irregular surface repartition permit their assignment to thermal explosion of impurities due to multiple exciton condensation.

Influence of temperature on crater and ejecta size following hypervelocity impact of aluminum alloy spheres on thick aluminum alloy targets

The influence of temperature on crater shape and ejecta fragment size in thick aluminum alloy targets was investigated for impact velocities ranging from approximately 0.9–3.5 km/s using a two-stage light-gas gun. The diameter, depth, and volume of the crater increased with increasing temperature. Observation of the witness plates showed that the scatter diameters and angles of the ejecta were slightly smaller at high temperatures than at low and room temperatures. Temperature affected the size and mass of ejecta fragments collected from the chamber only when the impact velocity was 1.5 km/s (the velocity at which the projectile fragmentation started). As the temperature increased, its influence on the mass of ejecta fragments decreased. Temperature affected the length and the axial ratios of ejecta fragments only at an impact velocity of 1.5 km/s. Regardless of the temperature and impact velocity, the axial ratios, c/a, were less than 0.6.

Brittle failure zone size under the concentrated charge blasting near free surface

The authors examine the blast action near free surface in brittle solid rocks, where fracture takes shape of crack development. The calculation model of kinematics of cracks initiated from the explosion center and extending onto free surface is worked out based on the linear brittle failure theory. To simplify the problem, cracks are assumed axially symmetric. The calculations involved specific parameters of a medium, charge and its installation depth. The calculations allow the shapes of cracks and their size-to-time relation, as well as the size and shape of explosion crater under the concentrated charge blasting near free surface.

Impact of spherical projectiles into a viscoplastic fluid

We study the behavior of a yield-stress fluid following the impact of a vertically falling sphere. Since the impact produces shear stresses larger than the yield stress, the material in the vicinity of the impact becomes fluidized. The sphere entrains air when it enters the fluid, and the resulting cavity pinches off below the surface. The upper part of this cavity then rebounds upward. For sufficiently fast impacts, a vertical jet is produced by the cavity collapse. While many aspects of this process are similar to that in Newtonian fluids or granular materials, the rheological properties of our target material change the scaling of the cavity pinch-off depth and have a dramatic effect on the height of the jets. The material returns to a solid-like behavior once the stresses due to the impact have relaxed to below the yield stress, leaving a crater in the surface of the material. We find that the diameter of this crater depends nonmonotonically on the impact speed. The crater shape also changes with speed, reflecting the dynamics of the impact process.

Performance of Different Projectile Nose Shapes in Normal Penetrating Armor Targets

Based on the cavity expansion theory, an analysis model of normal penetration of slender nose projectile into armor targets was established, and the variation relation curve of penetration depth and initial velocity when initial velocity is below ballistic limit was obtained. By carrying out ballistic impact test using 85mm smoothbore gun, various projectile nose shapes and armor target thickness and their effect on damage were considered. The research results have indicated that the damage of target is mainly ductile reaming and the shape of crater is almost the same as the projectile nose embedded in the target. Within a relatively low velocity range, nose shape has a greater influence on penetration depth. In contrast, within a relatively high velocity range, the target thickness has a greater influence on penetration depth. It can provide theoretical basis for the design of projectile noses and analysis of anti-penetration performance of armor targets.

Quantitative morphological evaluation of laser ablation on calculus using full-field optical coherence microscopy

The quantitative morphological evaluation at high resolution is of significance for the study of laser-tissue interaction. In this paper, a full-field optical coherence microscopy (OCM) system with high resolution of ∼2 μm was developed to investigate the ablation on urinary calculus by a free-running Er:YAG laser. We studied the morphological variation quantitatively corresponding to change of energy setting of the Er:YAG laser. The experimental results show that the full-field OCM enables quantitative evaluation of the morphological shape of craters and material removal, and particularly the fine structure. We also built a heat conduction model to simulate the process of laser-calculus interaction by using finite element method. Through the simulation, the removal region of the calculus was calculated according to the temperature distribution. As a result, the depth, width, volume, and the cross-sectional profile of the crater in calculus measured by full-field OCM matched well with the theoretical results based on the heat conduction model. Both experimental and theoretical results confirm that the thermal interaction is the dominant effect in the ablation of calculus by Er:YAG laser, demonstrating the effectiveness of full-field OCM in studying laser-tissue interactions.

Characteristics of large landslides in sensitive clay in relation to susceptibility, hazard, and risk

The clay plains of the Saint Lawrence Lowlands of eastern North America are subject to large landslides in sensitive clay. These landslides occur relatively infrequently, but can have very significant consequences. This type of risk (low frequency, high consequence) can be difficult to manage, as the return period is long enough that the most recent major event tends to be forgotten by the time the next major event occurs. This paper examines characteristics of large landslides in sensitive clay with the purpose of understanding the nature of the hazard, and this work is extended to develop a high level appreciation of risk to a network of linear infrastructure, using railways as an example. The analysis considers the characteristics of a number of large landslides documented in the literature, as well as statistical characteristics of a digital inventory of large landslides, including: surface area, debris travel distance, retrogression length, proximity to other landslides, crater shape, landslide mechanism, temporal frequency, and documented effects. A linear network with between 100 and 1000 river crossings in the sensitive clay deposits in eastern Canada is expected to suffer a major disruption once every 10 to 100 years.

The Study of Projectile Penetration into Aluminum Armor Plate

The experiments of certain aluminum alloy plate penetrated by 7.62mm armor piercing projectile were carried out, then a penetration model of aluminum alloy plate impacted by 7.62mm projectile was created by using ABAQUS software and Johnson-Cook constitutive equation, and the distribution of stress during penetration process was analyzed. Experimental and simulation results show that the penetration model can simulate the shape and depth of crater, and the outputs of stress and strain during penetration process can provide basis of the formation mechanism of adiabatic shear bands for aluminum alloys.

Finite Element Analysis of Ti-6Al-4V Alloy Crater Geometry for EDM Based on a Single Discharge

Temperature theoretical model of Ti-6Al-4Valloy during electrical discharge machining (EDM) process is established in this paper. Based on finite element thermal analysis, the temperature field of EDM Ti-6Al-4V on a single discharge is simulated with the aid of ANSYS software. The results show that there is a high temperature distribution around the spark point while the thermal-affected zone is quite small. Additionally, the profiles of discharge crater geometry have been predicted and the relation between crater shape and parameters under different current and pulse duration are also explored.

Damage of Ti-6Al-4V Alloy Induced by Hypervelocity Impact

The impact of macro- and microdamage behaviors of Ti-6Al-4V alloy has been examined using a two-stage light gas gun. Disks with thicknesses from 2 to 20 mm cut from a forged and annealed Ti-6Al-4V alloy bar with the diameter of 70 mm, and plates cut from a rolled Ti-6Al-4V alloy sheet with a thickness of 1 mm, were employed as single- and multilayer targets, respectively. The impact velocities of the spherical steel, 4.0-mm-diam projectiles, ranged from 1 to 4 km/s. For the impacted targets, the crater shape and dimension, microstructures and microhardness distributions in the vicinity of craters were characterized by complementary techniques such as optical microscopy, scanning electron microscopy, and Vickers microhardness measuring. Based on the experimental results, the penetration resistance of the annealed Ti-6Al-4V alloy and the critical impact velocity to form the debris clouds in the Ti-6Al-4V alloy sheet were obtained.

Plasma characterization of dry μ-EDM

An attempt was made to characterize the dry microelectric discharge machining (μ-EDM) plasma systematically by using optical emission spectroscopy. In order to characterize the plasma and its parameters such as plasma temperature, electron density, Debye length, and gamma parameter, the optical spectra were recorded for different energies and with different diameters of tool electrode. From the recorded spectra, using line pair method and modified Saha equation, plasma temperature and electron density, respectively were calculated. From these two, Debye length and plasma parameter were also calculated. These studies indicate that the plasma produced in dry μ-EDM is ideal. In addition to this, we found that the dry μ-EDM region is below the conventional EDM region in log electron density and log temperature plot. Morphological analysis was carried out on crater produced by different energies on the workpiece using a scanning electron microscope. The crater shape may dependent on the geometry of the tool tip, number of discharges, and energy of discharge.

Electrical properties of the µs pulsed glow discharge in a Grimm-type source: comparison of dc and rf modes

The electrical properties, in particular the U–I characteristics, current and voltage signal shapes within the pulse, are important parameters for the understanding of the processes taking place in the pulsed glow discharge (PGD). The electrical properties are also closely related to the analytical performance of the PGD such as sputtering rates, crater shapes and emission yields. Moreover, the dependence of the U–I plots on the density of the discharge gas can be used to estimate the gas temperature. This result is relevant for the analysis of thermally fragile samples. Nevertheless, there is a lack of PGD studies where the current and voltage signals are considered in detail. Therefore, this article is dedicated to the electrical properties of PGD. The influence of the PGD parameters (duty cycle and pulse duration) on the electrical properties is examined. The results highlight the optimum parameters for particular analytical applications. The question, whether direct current (dc) and radio frequency (rf) discharges behave similarly is also discussed and all experiments are performed for both modes. The comparative studies reveal strong similarities between dc and rf pulsed discharges.

Analytical performance of pulsed radiofrequency glow discharge optical emission spectrometry for bulk and in-depth profile analysis of conductors and insulators

Radiofrequency glow discharge (rf-GD) coupled to optical emission spectrometry (OES) has proved to be a powerful tool for the direct solid bulk materials and in-depth profile analysis. Rf-GD is generally operated in continuous mode, but the interest in pulsed (P) rf-GD is steadily increasing. So far, however, investigations to assess the analytical performance of rf-PGD-OES are scarce and systematic studies to analyse homogeneous and/or coated samples are still lacking. This work aims at critical comparison of the practical analytical performance characteristics of rf-GD-OES operated in continuous and pulsed modes. Pulsed rf-GD was carefully evaluated for the analysis of three conducting materials with different matrices (copper, steel and aluminium) and for bulk homogenous non-coated glasses of varying thicknesses (1, 1.8 and 2.8 mm). The comparison was performed in terms of crater shapes, sputtering rates and emission yields, demonstrating that increased emission yields could be obtained in pulsed mode working with small pulse widths (i.e. at high pulse frequency and relatively short duty cycle). Further, the potential of the pulsed rf-GDs has been studied also for depth profiling analysis of a thin gold layer (∼30 nm) deposited on conductive and insulating substrates. An improvement in the depth-resolution, under selected experimental conditions, was clearly observed. Finally, those observed advantages of pulsed rf-GD-OES were successfully tested by analysing real-life samples including commercial tinplates and a multilayered glass.

Molecular dynamics simulations of large fluorine cluster impact on silicon with supersonic velocity

The etching process by very large reactive gas cluster impact was investigated by molecular dynamics (MD) simulations. Fluorine-molecule clusters with the size up to 100,000atoms (50,000 F2 molecules) were irradiated on silicon (100) targets at supersonic velocity regime (0.1–1eV/atom, 1.0–3.2km/s). The MD simulations revealed that the existence of threshold energy-per-atom around 0.3eV/atom (1.75km/s) to cause surface deformation and enhancement of Si desorption. When the incident energy-per-atom is less than the threshold, the incident cluster breaks up itself on the target without surface deformation. The fluorine molecules in the cluster spread in the lateral direction along the target surface, and some part of them decompose and adsorbs on the target to form silicon fluoride composites. On the other hand, the clusters penetrate the surface of silicon target when the energy-per-atom is larger than 0.3eV/atom. In these collisional processes, the target surface is deformed to create shallow crater shape. The incident fluorine molecules are preferentially concentrated at the bottom of the crater, which resulted in high desorption yield of silicon as in the form of SiF2, SiF3 and SiF4.

Intelligent process modeling and optimization of die-sinking electric discharge machining

This paper reports an intelligent approach for process modeling and optimization of electric discharge machining (EDM). Physics based process modeling using finite element method (FEM) has been integrated with the soft computing techniques like artificial neural networks (ANN) and genetic algorithm (GA) to improve prediction accuracy of the model with less dependency on the experimental data. A two-dimensional axi-symmetric numerical (FEM) model of single spark EDM process has been developed based on more realistic assumptions such as Gaussian distribution of heat flux, time and energy dependent spark radius, etc. to predict the shape of crater, material removal rate (MRR) and tool wear rate (TWR). The model is validated using the reported analytical and experimental results. A comprehensive ANN based process model is proposed to establish relation between input process conditions (current, discharge voltage, duty cycle and discharge duration) and the process responses (crater size, MRR and TWR) .The ANN model was trained, tested and tuned by using the data generated from the numerical (FEM) model. It was found to accurately predict EDM process responses for chosen process conditions. The developed ANN process model was used in conjunction with the evolutionary non-dominated sorting genetic algorithm II (NSGA-II) to select optimal process parameters for roughing and finishing operations of EDM. Experimental studies were carried out to verify the process performance for the optimum machining conditions suggested by our approach. The proposed integrated (FEM–ANN–GA) approach was found efficient and robust as the suggested optimum process parameters were found to give the expected optimum performance of the EDM process.